Australian scientists have confirmed the oldest penis-like structure in an ancient fish specimen.

The discovery of the 400 million-year-old reproductive organ is one of the earliest examples of internal fertilisation in vertebrate animals.

And understanding the anatomy of these ancient fish could reveal further details in the evolution of vertebrates- including humans.

The research is published in today's advanced online ahead of print edition of Nature.

Earlier this year the team, led by Australian palaeontologist Dr John Long, predicted some ancient fish from the Devonian era, had an attachment to their pelvic bone, which were used by males to fertilise females.

Long, of Museum Victoria, says "when we announced we'd found some structures in the pelvic fin that suggested copulation, we hadn't found the business end of how they were doing it."

Now the team have identified a long clasper, made entirely of bone, on another fish specimen.

Long says claspers were used by the ancient fish, an extinct class of armoured fish called placoderms, to grip inside the female while they were mating.

"It's a pretty big find because placaderms were the dominant fish for 70 million years, but we knew nothing about their reproduction," says Long.

Similar to sharks

He says their work earlier this year suggests the reproductive structure in the dominant group of placoderms, called arthrodires, was similar to present day sharks.

"Now we've actually found it, a specimen with an undoubted clasper with a knobbly end."

Study author and palaeontologist Dr Kate Trinajstic, of Curtin University in Perth, says the clasper was discovered in a fish specimen uncovered in the Gogo region of Western Australia in 2001.

She says the team originally discounted the bone as the reproductive organ because they thought it was part of the pelvic gurdle.

On closer inspection, Trinajstic says they realised it was a sexual organ.

"We were surprised because it's so big," she says. "We were expecting something smaller."

Trinajstic says the clasper, which was attached to the pelvic organ would have been erectile.

"It penetrates the female, and acts like a funnel, allowing the transfer of sperm."

She says the ancient fish had quite advanced reproductive systems considering sharks today have a similar system.

Trinajstic says the discovery of the clasper now allows scientists to identify the sex of other specimens of ancient fish.

"That sounds like a basic thing, but we haven't been able to do that before."